University of Groningen

Ancillary ligand effects in organoyttrium chemistryDuchateau, Robbert

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41

The Reactivity of Bis(Benzamidinato) Yttrium Alkyl

and Hydrido Compounds.∗∗

3-1. Introduction.

Recent reactivity studies on well-defined group 3 and lanthanide carbyl and hydrido

derivatives show a very rich and diverse chemistry.1 Dominant reaction pathways are insertion

of unsaturated substrates into Ln-X (X = C, H, N) bonds and X-H (X = C, H, N, O) bond

activation. In the latter case, the activation of saturated hydrocarbons like methane is amongst

the most intriguing.1c,e,2 Although some modifications of the ligand system have been

introduced,3,4 the most extensively studied system is Cp*2LnR (Ln = Sc, Y, lanthanides; R =

alkyl, hydride).1

Our interest in this field of chemistry is aimed at obtaining a better understanding of how

and to what extent electronic and steric properties of ancillary ligands influence the reactivity of

a complex. With the bis(N,N’-bis(trimethylsilyl)benzamidinato) yttrium alkyl (2.6, 2.7, 2.8) and

hydrido (2.9) complexes available (Chapter 2), we decided to explore their potential in C-X

activation and insertion reactions. To get a good picture of the similarities and differences in

reactivity of these new organoyttrium compounds compared with the known chemistry, the

choice of substrates was limited to those previously used to test the reactivity of the well

characterized yttrocene and analogous systems,1-3 Cp*[ArO]YR (ArO = O-2,6-

(CMe3)2C6H3)4d and [OEP]YR (OEP = octaethylporphyrin)4c (R = alkyl, hydride).

3-2. Thermolysis and Behavior of [C6H5C(NSiMe3)2]2YR (2.6 - 2.9) in Common

Solvents.

Before the reactivity of these compounds towards unsaturated substrates was investigated,

their behavior and thermolysis in common solvents was tested. When reactivity studies are

carried out and experimental data interpreted, the possibility of side reactions such as thermal

∗ Parts of this work were performed in collaboration with C. T. van Wee and E. A. Bijpost. The X-ray structure

determination described in this chapter was carried out by A. Meetsma (University of Groningen).

3

Chapter 3.

42

decomposition, C-X (X = H, hetero atom) of the solvent or ligand exchange should be taken

into account.

Aromatic and Aliphatic Solvents. At room temperature, the alkyl complexes

[C6H5C(NSiMe3)2]2YR ( = (µ-Me)2Li.TMEDA (2.6), CH2Ph.THF (2.7), CH(SiMe3)2 (2.8)) and

the hydride {[C6H5C(NSiMe3)2]2Y(µ-H)}2 (2.9) are stable in aromatic and aliphatic hydrocarbon

solvents (several months). Even at elevated temperatures (100oC), they remain intact for

prolonged periods of time (hours to days). This stability is quite surprising when compared with

their Cp*2LnR analogues (R = alkyl, H; Ln = Sc, Y, lanthanides) which undergo metalation of

the Cp* ligands upon thermolysis, affording fulvene species.1c,d-g,n,r, 5 Another interesting

feature of the bis(N,N'-bis(trimethylsilyl)benzamidinato) yttrium alkyls and hydride is their

complete lack of H/D scrambling and metalation of aromatic solvents; a dominant feature of

Cp*2LnR systems.1

Ethereal Solvents. The alkyls 2.6, 2.7 and 2.8 are stable in ethereal solvents (Et2O, THF;

room temperature). In contrast to {Cp*2Y(µ-H)}2, the hydride {[C6H5C(NSiMe3)2]2Y(µ-H)}2

(2.9) is stable in diethylether (2 weeks 25oC).1e In benzene, 2.9 does not react with THF (2

equiv.) but in neat THF (hours at room temperature), ethane and minor amounts (± 15 %) of

ethylene (Töpler pomp determination, GC-MS) are formed. Olefinic resonances in the 1H

NMR spectrum of the product mixture suggest the formation of yttrium enolate species as a

result of C-O bond activation.

Scheme 1.

RM +O

C

C

O H

H H

M OM MHM

- ethene (C)

O(CH2)4

HM OM MHM

- butane(B)

OM C

C

O H

H H

M- ethene (A)

- ethane

- R-H

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

43

Examples of C-O bond activation and metalation of THF by group 3 metal and lanthanide alkyls and

hydrides: (A): M = [C5H4Me]2Y, R = CH3.THF, CH2SiMe3.THF. (B): M = Cp*2Ln (Ln = Y, Sm), R = µ-

H. (C): M = Cp*2Ce, [C6H5C(NSiMe3)2]2Y, R = µ-H.

Cleavage of THF is known to proceed by a variety of reaction routes (Scheme 1). For BuLi6

and [C5H4Me]2YR.THF (R = Me, CH2SiMe3),7 ortho -metalation followed by extrusion of

ethylene has been proposed (Scheme 1A). The hydrides {Cp*2Ln(µ-H)}2 (Ln = Y, Sm) are

known to activate the C-O bond yielding the butoxo derivatives Cp*2Ln-O-nBu1n,8 In an

analogous reaction, [Cp*2Sm.THF2]+[BPh4]- reacts with Cp*K to form Cp*2Sm-O-

(CH2)4C5Me5.THF (Scheme 1B).9 In contrast, with THF the closely related {Cp*2Ce(µ-H)}2

yields a µ-oxo species with elimination of ethane (0.5 mol per mol Ce; Scheme 1C).8 An

unusual cleavage of THF was observed by Gambarotta et al. who isolated a vanadium

ynolate species on treatment of VCl3.THF3 with [OEPG]Li4.THF4 (OEPG =

octaethylporphyrinogen).10 The formation of ethane (quantitative) and of minor amounts of

ethylene (no hydrogen), in combination with the observed olefinic proton resonances in the 1H

NMR spectrum of the product mixture, suggest that the reaction of 2.9 with THF proceeds by

the same mechanism as proposed for {Cp*2Ce(µ-H)}2. The difference in product distribution,

compared to the reaction of {Cp*2Ce(µ-H)}2 with THF, indicates that the second C-O bond

activation, yielding the corresponding µ-oxo species, is slower in our system.

When reactivity studies are carried out and experimental data interpreted, the possibility of

side reactions, such as ligand exchange, should be taken into account. The reaction of the

chloride 2.2 with LiAlH4 demonstrated that under certain conditions, benzamidinato ligands

exchange rapidly (Cf. Chapter 2).

3-3. Reactivity Towards Alkenes.

Olefin Polymerization. Migratory olefin insertion into metal-carbon, metal-hydrogen1e,3a,e,4

and even metal-amido1o,3d bonds is a fundamental step, essential to catalytic processes

involving olefins. The capability of group 3 and lanthanide elements to catalyze these

transformations is clearly demonstrated by the fact that many of their well-defined

organometallic compounds are catalysts for olefin hydrogenation and polymerization with

activities approaching, or even exceeding, those of homogeneous systems based on other

transition metals.1c,3a,b

Chapter 3.

44

When benzene-d6 solutions of 2.9 were treated with ethylene (4 atm) no reaction was

observed. However, heating to 65oC resulted in the formation of polyethylene.11 1H NMR

spectra collected during the reaction showed the gradual consumption of ethylene and more

importantly 2.9 as the only organoyttrium compound present in detectable amounts. This

suggests that dissociation of 2.9 into the active monomer (initiation) is much slower then

propagation (Figure 1). This is supported by extensive studies of group 3 metal and f-element

catalyzed olefin insertion processes for which it has been found that unsolvated, monomeric

compounds are much more reactive than the corresponding oligomeric or solvated

compounds.1g,2,3e,4c,d Resonances attributable to ethylene oligomers or mixed hydrido-ethyl

species, [Y](µ-H)(µ-Et)[Y] ([Y] = [C6H5C(NSiMe3)2]2Y), as reported by Marks et al. for [Y] =

[µ,η5,η5-(C5Me4)SiR2(C5H4)]2Y3e and by Schaverien for [Y] = Cp*[ArO]Y,4d were not

observed. This indicates that propagation is fast relative to the rate of termination. Since the

affinity of 2.9 towards ethylene is low, reactions under more drastic conditions were carried out

(55oC, 70 atm ethylene). Although some increase in activity was found (4 g PE /

mmol(2.9).h),11 the total yield of polyethylene remained low.12

Y

N

N

NN R

Y

N

N

N THFN CH2Ph

Y

N

N

NN

HH

Y

N

N

NN

- THF

H2C CH2

R = H, CH2Ph

Polyethylene2.9

2.7

Figure 1. Slow dissociation of the dimeric hydride (2.9) or of the coordinated THF in 2.7, necessary

to create an open coordination site for an incoming ethylene molecule, is believed to inhibit the

reaction with ethylene.

To circumvent the inhibiting effect of hydrido bridges, the reaction of the monomeric

[C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) with ethylene was carried out. At 65oC (4 atm

ethylene), 2.7 polymerizes ethylene, although the rate seems to be lower than found for the

reaction of 2.9 with ethylene under the same conditions. Again, the 1H NMR spectra collected

during and after the reaction showed the gradual consumption of ethylene and 2.7 as the only

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

45

detectable organoyttrium derivative present. Like dissociation of 2.9 into the monomeric

hydride, replacement of the coordinated THF molecule in 2.7 by an incoming ethylene

molecule is assumed to be the rate determining step.

Surprisingly, with propylene (4 atm) and 1-hexene (neat), the precursors 2.7 and 2.9 were

left unreacted. Neither oligo-/polymerization or indication for the formation of an η3-allyl

species, as found for {Cp*2Ln(µ-H)}2 (Ln = Y, lanthanides),1c,f,13 was observed under the

conditions employed (days, 80oC).

Olefin Hydroboration. As part of a more general study on the hydroboration of alkenes

catalyzed by group 3 and 4 metals and lanthanides, the catalytic activity of the yttrium

bis(benzamidinato) compounds 2.7, 2.8 and 2.9 in hydroboration of 1-hexene with

catecholborane was investigated.14 All complexes proved to be catalytically active, but the

activity is about 5 - 10 % of that of the lanthanum complex, Cp*2LaCH(SiMe3)2, for which the

highest activity has been reported.1p

O

O

BH

HN

N

N

N

Y

Figure 2. Proposed intermediate for the hydroboration of 1-hexene, catalyzed by 2.7 - 2.9.

In all cases, fast catalyst deactivation occurs, probably due to the reaction of catecholborane

with the benzamidinato ligands. A possible explanation is transfer of the benzamidinato

ligands from yttrium to boron, comparable to the observed ligand exchange reaction of

[C6H5C(NSiMe3)2]2YCl.THF (2.2) with LiAlH4 (Chapter 2). Nevertheless, 2.8 and 2.9 showed

a substantially higher catalytic activity than Cp*2YCH(SiMe3)2.14 This is surprising, taking into

account that in the absence of catecholborane no reaction of 2.9 with 1-hexene was observed

at all. Possibly, catecholborane activates the yttrium catalyst by forming intermediates such as

shown in Figure 2. It could also be that transfer of ligands from yttrium to boron initially

activates the catalyst. A more extensive discussion concerning the hydroboration of α-olefins

catalyzed by the yttrium bis(benzamidinato) system, including experimental details, has been

given elsewhere.14

Chapter 3.

46

3-4. Reactivity Towards 1-Alkynes.

Selective dimerization of terminal alkynes, catalyzed by various transition metals has been

extensively studied.15 Until a few years ago, synthetic application of the catalytic dimerization

of 1-alkynes had been limited due to the lack of selectivity and the formation of higher

oligomers. Recently, a number of d-block metal and lanthanide complexes have been shown

to be catalysts for the selective dimerization of 1-alkynes. Typical examples are

[P(CH2CH2PPh2)3]RuH2,16 Cp*2TiR and Cp*2TiCl2/RMgX,17 [{Cp*2ZrMe}+{B(4-C6H4F)4}-],18

Cp*2LnR1d,e,k and Cp*[ArO]YR.4c The first step in the catalytic cycle is believed to be

formation of a metal alkynyl, [M]-C≡CR.

Since terminal alkynes are more reactive than olefins, it was anticipated that this substrate

would be ideal to probe the catalytic potential of our system. A study concerning the

dimerization of 1-alkynes catalyzed by bis(benzamidinato) yttrium complexes will be discussed

below (Section 3-5.). First the synthesis and characterization of some well-defined alkynyl

derivatives will be described.

Synthesis of Bis(N,N-Bis(Trimethylsilyl)Benzamidinato) Yttrium Alkynyl Derivatives.

[C6H5C(NSiMe3)2]2YCH(SiMe3)2 (2.8) reacts with one equivalent of several terminal alkynes,

HC≡CR, to give the corresponding dimeric acetylides, {[C6H5C(NSiMe3)2]2Y(µ-C≡CR)}2 (R =

H (3.1), Me (3.2), n-Pr (3.3), SiMe3 (3.4), Ph (3.5), CMe3 (3.6), eq. 1) in nearly quantitative

yield.

3.1, R = H; 3.2, R = Me;3.3, R = n-Pr; 3.4, R = SiMe3;3.5, R = Ph; 3.6, R = CMe3.

Y

N

N

N

N

H

SiMe3

SiMe3

C Y

N

N

NN

CC

Y

N

N

NN

CR

CR

(1)

2.8

HC CR

- H2C(SiMe3)2

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

47

With the hydride 2.9 as starting material for 3.6, minor amounts of the intermediate

[{[C6H5C(NSiMe3)2]2Y}2(µ-H)(µ-C≡CCMe3)] (1H NMR: δ = 8.0 (t, µ-H, 1JYH = 28 Hz)) and

traces of H2C=C(H)CMe3 were observed in the 1H NMR. The presence of this intermediate

indicates a stepwise reaction of 2.9 with HC≡CCMe3 (eq. 2), comparable to the reaction of

{Cp*[ArO]Y(µ-H)}2 with HC≡CSiMe3.4d Traces of H2C=C(H)CMe3 indicate hydrogenation of

the terminal alkyne. When 3.6 was treated with dihydrogen no H2C=C(H)CMe3 could be

observed. Hence, the hydrogenation of 3,3-dimethyl-1-butyne is catalyzed by either 2.9 or

[{[C6H5C(NSiMe3)2]2Y}2(µ-H)(µ-C≡CCMe3)]. This is very similar to the reaction of

[C6H5C(NSiMe3)2]2AlH with HC≡CCMe3, where in addition to C-H bond activation of

HC≡CCMe3 affording [C6H5C(NSiMe3)2]2Al-C≡CCMe3, a competing insertion reaction of

HC≡CCMe3 into the Al-H bond was observed, yielding H2C=C(H)CMe3 and

[C6H5C(NSiMe3)2]2Al-C≡CCMe3. This aluminum alkynyl formed showed no further reactivity

towards HC≡CCMe3 and/or dihydrogen.19

Y

N

N

NN

HH

Y

N

N

NN

2.9

Y

N

N

NN

HC

Y

N

N

NN

C

Y

N

N

NN

CC

Y

N

N

NN

C

C

(2)

3.6

HC CCMe3

- H2

HC CCMe3

- H2

The 13C NMR spectra (benzene-d6) of the yttrium acetylides (3.1 - 3.6) at elevated

temperatures (70-80oC) did not show any sign of dissociation, suggesting stable dimeric

structures. However, the dimeric acetylides can be cleaved by a Lewis base. Addition of THF

to solutions of 3.1 or 3.6 resulted in instantaneous formation of the corresponding monomeric

THF adducts, [C6H5C(NSiMe3)2]2YC≡CR.THF (R = H (3.7); R = CMe3 (3.8), eq. 3). This

corresponds with the reaction of {Cp*2Ce(µ-C≡CCMe3)}n with THF, affording

Cp*2CeC≡CCMe3.THF.8 In contrast, the compounds [C5H4R]2Y(µ-C≡CCMe3)}2 (R = H, Me)

Chapter 3.

48

does not react with THF.1c The monomeric 3.8 could also be isolated when 2.7 was treated

with HC≡CCMe3 (eq. 4).

3.1, R = H;3.6, R = CMe3.

3.7, R = H;3.8, R = CMe3.

Y

N

N

N THF

N C CRY

N

N

NN

CC

Y

N

N

NN

CR

CR

THF(3)

Y

N

N

N THF

N C CY

N

N

N THF

N CH2Ph (4)

2.7 3.8

HC CCMe3

- toluene

Finally, when the ‘ate’ compound, [C6H5C(NSiMe3)2]2Y(µ-Me)2Li.TMEDA (2.6), was treated

with HC≡CCMe3 the corresponding [C6H5C(NSiMe3)2]2Y(µ-C≡CCMe3)2Li.TMEDA (3.9) could

be isolated. This compound is structurally comparable with Cp*2Y(µ-C≡CCMe3)2Li.THF and

[Cp*2Sm(µ-C≡CPh)K]n, prepared by Evans et al. ,20 and Cp*[C6H5C(NSiMe3)2]Y(µ-

C≡CCMe3)2Li.TMEDA which will be discussed in Chapter 6.

2.6 3.9

Y

N

N

N Me

N Me Li

N

N

Y

N

N

N C

N C Li

N

NC

C

(5)HC CCMe3

Characterization. The IR spectra of 3.1-3.9 show ν(C≡C) stretching vibration frequencies

ranging from 1914 to 2060 cm-1, comparable to those of other dimeric group 3 and lanthanide

acetylides1d,e,k, 4d,20,21 and notably lower than those of the corresponding free acetylenes.22

The 1H NMR spectra (25oC) of 3.1 - 3.9 show sharp singlets for the benzamidinato

trimethylsilyl groups, indicating fast fluxional behavior in solution. The 13C NMR spectra of 3.1

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

49

- 3.6 display a triplet (20-21 Hz) for the α-carbon of the acetylide. The splitting of the signal is

due to 89Y-13C coupling and indicates that each alkynyl ligand interacts with two (time-

averaged) identical yttrium centers. It is remarkable that the α-C resonance of 3.4 (δ = 172.3

ppm, 1JY-C = 19 Hz) is considerably shifted to low-field compared to 3.1 - 3.3 and 3.5 - 3.6 (δ

ranges from 136.0 to 146.6 ppm); why this should be so is, as yet, unclear. The 13C NMR

spectra of 3.7 - 3.9 show a doublet (3.7: δ = 111.6 ppm, 1JY-C = 11 Hz; 3.8: δ = 89.3 ppm,1JY-C

= 12 Hz; 3.9: δ = 121.1, 1JY-C = 15 Hz) for the α-carbon of the acetylide, proving that the

complexes are monomeric. Worth mentioning is that, due to the coupling with yttrium, the β-H

of the ethynyl ligand of 3.7 appears as a doublet in the 1H NMR spectrum (δ = 1.99 ppm, 3JY-H

= 1.3 Hz). This suggests a stronger interaction of yttrium with the ethynyl fragment in the

monomeric 3.7 than in the dimeric 3.1, for which no yttrium coupling is observed. However,

the yttrium-carbon coupling constants in these compounds (3.7: 1JY-C = 11 Hz, 3.1: 1JY-C = 20

Hz) suggest the opposite.

Table I. Selected Bond Distances and Angles for {[C6H5C(NSiMe3)2]2Y(µ-C≡CH)}2 (3.1).

Distances (Å).

Molecule A. Molecule B.

Y(3)-N(5) 2.395(4) Y(1)-N(1) 2.396(4)

Y(3)-N(6) 2.345(4) Y(1)-N(2) 2.350(4)

Y(3)-N(7) 2.335(4) Y(2)-N(3) 2.376(4)

Y(3)-N(8) 2.381(4) Y(2)-N(4) 2.343(4)

Y(3)-C29) 2.556(5) Y(1)-C(1) 2.503(6)

Y(3)a-C(29) 2.509(5) Y(2)-C(1) 2.558(6)

Y(3)-C(30 2.959(10) Y(2)-C(2) 2.970(10)

N(5)-C(31) 1.329(6) N(1)-C(3) 1.333(6)

N(6)-C(31) 1.329(6) N(2)-C(3) 1.325(7)

N(7)-C44) 1.320(6) N(3)-C(16) 1.337(6)

N(8)-C(44) 1.337(6) N(4)-C16) 1.318(6)

C(29)-C(30) 1.164(8) C(1)-C(2) 1.148(14)

Angles (deg)

Molecule A. Molecule B.

N(5)-Y(3)-N(8) 149.33(15) N(1)-Y(1)-N(1)b 148.78(15)

N(6)-Y(3)-N(7) 98.70(14) N(2)-Y(1)-N(2)b 101.32(14)

Chapter 3.

50

N(5)-Y(3)-N(6) 57.92(13) N(1)-Y(1)-N(2) 57.90(13)

N(7)-Y(3)-N(8) 58.12(13) N(3)-Y(2)-N(4) 58.00(13)

Y(3)-C(29)-C(30) 98.4(5) Y(1)-C(1)-C(2) 158.3(9)

Y(3)a-C(29)-C(30) 159.8(5) Y(2)-C(1)-C(2) 99.4(9)

C(31)-Y(3)-C(44) 119.75(15) C(3)-Y(1)-C(3)b 120.55(15)

C(16)-Y(2)-C(16)b 118.87(15)

N(5)-Y(3)-N(6)-C(31) -3.8(3) N(1)-Y(1)-N(2)-C(3) -3.2(2)

N(7)-Y(3)-N(8)-C(44) -3.3(3) N(3)-Y(2)-N(4)-C(16) -4.1(2)

Symmetry operations: a = -x, y, ½ -z; b = -x, y, ¾ -z.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

51

Molecular Structure of {[C6H5C(NSiMe3)2]2Y(µµ-C≡≡CH)}2 (3.1). To determine the bonding

of the alkynyl fragment in 3.1, an X-ray crystal structure determination was carried out.23 The

unit cell contains two crystallographically independent molecules (both exhibiting C2

symmetry) that do not differ markedly in their structure. PLUTO drawings of both molecules

are depicted in Figure 3.

+-[Y][Y]

C

C

C

H

C

H

HC

C

[Y]

HC

C[Y]

3.1A 3.1B

Figure 3. PLUTO drawings and schematic presentations of both crystallographically independent

molecules of {[C6H5C(NSiMe3)2]2Y(µ-C≡CH)}2 (3.1). For clarity reasons, only the ethynyl hydrogens

are shown.

In molecule 3.1A (Figure 3A), two identical [C6H5C(NSiMe3)2]2Y-C≡CH units are related to

each other by a 2-fold axis perpendicular to the plane of the acetylide bridge. In the other

molecule (3.1B, Figure 3B), both yttrium atoms are on a special position along the 2-fold axis.

Chapter 3.

52

In molecule 3.1A, each yttrium is σ-bonded to one ethynyl ligands and π-bonded to the other.

As a result of the symmetry operation obeyed by molecule 3.1B, both alkynyl ligands are σ-

bonded to Y(1), whereas the π-system of these alkynyls interact with Y(2). Hence, molecule

3.1B is best described as a Zwitterion, {[C6H5C(NSiMe3)2]2Y(C≡CH)2}-

{Y[C6H5C(NSiMe3)2]2}+ (Figure 3), which is in agreement with the longer Y(2)-C(1) (2.558(6)

Å) compared to the Y(1)-C(1) bond (2.503(6) Å).

Since both the benzamidinato-SiMe3 and ethynyl groups emerge as single resonances in

the 1H and 13C NMR spectra, fast equilibration of the regio-isomers (3.1A, 3.1B) occurs in

solution (Figure 3). Erker et al. found a ∆Go of approximately 0.1 kcal.mol-1 for a similar

equilibrium for dimeric zirconium alkynyl derivatives, [C5H5]2Zr(µ-C≡CMe)2 Zr[C5H4CMe3]2.24

As the majority of the corresponding bond lenghts and angles are very similar in both

molecules, only notable features of molecule 3.1A will be discussed here. Molecule 3.1A is

formed by two distorted octahedral yttrium atoms, each surrounded by two chelating

benzamidinato ligands and two bridging acetylide fragments. The Y(3)-N(5) (2.395(4) Å) and

Y(3)-N(8) (2.381(4) Å) bonds are longer than Y(3)-N(6) and Y(3)-N(7) (2.345(4) Å and

2.335(4) Å respectively). The virtually identical C-N bonds (Table I) of the benzamidinato

fragments indicate the usual electron delocalization within the NCN fragment. All Y-N

distances are within the range observed for other yttrium N,N’-bis(trimethyl-

silyl)benzamidinato derivatives (Chapter 2) and suggest some π-interaction with the yttrium

center. The Y(3)-C(29) (2.556(5) Å) and Y(3)a-C(29) (2.509(5) Å) bonds are longer than the

Y-C bond in Cp*2Y(µ-C≡CCMe3)2Li.THF (2.38(2) Å),20a but very similar to the Y-C distances

in [Cp2Y(µ-Me)]2 (2.553(1) Å, 2.537(9) Å).25 The Y-C bonds for 3.1A are within the range of

Ln-C bonds of dimeric 4f element acetylide complexes when the differences in atomic radii are

taken into account.21 The Y(3)-C(29) and Y(3)a-C(29) bonds and the Y(3)-C(29)-C(30)

(98.4(5)o) and Y(3)a-C(29)-C(30) (159.8(5)o) angles show that the bridge is non-symmetric but

still with a strong interaction of the α-carbon atom, C(29), with both yttrium centers. The

acetylide stretching vibration of ν(C≡C) = 1915 cm-1 (cf. HC≡CH: ν(C≡C) = 1947 cm-1;

Cp3UC≡CH: ν(C≡C) = 2062 cm-1),26 the large 1JC-H coupling constant (218 Hz) for Y-C≡CH and

the Y(3)a-C(29)-C(30) angle of 159.8(5)o suggest an interaction of the triple bond π-orbitals

with Y(3). Since π-interaction of the acetylide is expected to result in elongation of the C≡C

bond, the observed value of the C(29)-C(30) bond (1.164(8) Å), which is considerable shorter

than in free acetylene (1.21 Å),26a is surprising. Comparable short C≡C bonds27 were also

observed in alkali-metal alkynyls (NaC≡CMe (1.09(2) Å), NaC≡CH (1.17(6) Å)),27b Cp*2Sm-

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

53

C≡CPh.THF (1.11(2) Å, 1.13(2) Å),27c and [OEPG]V(OC≡CH)Li2.THF3 (1.102(9) Å),10 but the

reason for this shortening of the C≡C bond is unclear.

Thermolysis of Dimeric Acetylide Derivatives. Thermolysis (24-72 h, 100oC) of the

dimeric alkynyl species (3.1 - 3.6) resulted in the formation of several unidentified products.

The rate of the thermolysis strongly depends on the alkynyl substituent. 3.1 decomposes

within hours at 100oC, while the characteristic triplet signal for the alkynyl α-carbon of 3.4 is

still present after two days at 100oC. Unequivocal assignment of resonances in the NMR

spectra arising from C-C coupled µ-trienediyl compounds, as formed upon thermolysis of

{Cp*2Ln-C≡CR}n (Ln = La, Ce, Sm),28 was not possible. Although the β-H signal in the 1H

NMR of 3.1 disappeared upon thermolysis, clean formation of an acetylenediyl species similar

to Cp*2Ln-C≡C-LnCp*2 as found by Bercaw (Ln = Sc)29a and Evans (Ln = Sm)29b was not

observed in our system. Since a complicated mixture of very soluble thermolysis products

was formed, isolation and characterization of the individual products proved to be impossible.

3-5. Catalytic Dimerization of Terminal Alkynes.

To probe the catalytic activity of bis(benzamidinato) yttrium compounds in 1-alkyne

dimerization, a variety of substrates were examined and the results compared with those

obtained from Cp*2Ln-R (Ln = Sc,1e,29a Y,1d,k La,1k,28b Ce,1k,28b Sm;20b R = alkyl, alkynyl, H)

and Cp*[ArO]YR (R = alkyl, alkynyl)4d systems.

Scope of the Catalytic 1-Alkyne Dimerization Reaction. The dimerization of 1-alkynes

catalyzed by bis(N,N’-bis(trimethylsilyl) benzamidinato) yttrium alkyl and hydrido derivatives

(2.8, 2.9) strongly depends on the alkyne substituent. For small 1-alkynes, HC≡CR (R = H,

Me, nPr), no dimerization is observed (days, 80oC) and the reaction stops on formation of the

dimeric acetylides (3.1 - 3.3). It is only with acetylenes containing bulky substituents, HC≡CR

(R = Ph, SiMe3, CMe3), that catalytic dimerization takes place (vide infra ). This observation

strongly contrasts with the Cp*2LnR catalyzed dimerization of 1-alkynes, for which high activity

was observed for all alkynes applied regardless the size of the alkyne substituents. The

assumption that the first step in the dimerization is formation of the acetylide, is supported by

the fact that the reaction rate is independent from whether 2.8, 2.9 or the acetylides 3.4 - 3.6

are used as precursors.

Chapter 3.

54

Compared to the Cp*2LnR (Ln = La, Ce) catalyzed dimerization of 1-alkynes (Nt (25oC) ≥

1900 mol.mol-1.h-1),1k,28b the observed catalytic activity of [C6H5C(NSiMe3)2]2Y-R (R =

CH(SiMe3)2, µ-H, µ-C≡CR) in 1-alkyne dimerization, is much lower (Nt (80oC) = 20 - 40

mol.mol-1.h-1). Although the coordinated THF retards the reaction considerably, the THF

adduct 3.8 still does catalyze the dimerization of HC≡CCMe3 (Nt (50oC) ≈ 1 mol.mol-1.h-1),

which implies that the coordinated THF can be replaced by an incoming acetylene.

Table II. Product distribution of the oligomerization of terminal alkynes, HC≡CR, catalyzed by group

3 metal and lanthanide based complexes.

Compound R 2,4-dimer 1,4-dimer higher oligomers Ref.

[C6H5C(NSiMe3)2]2YR’ CMe3 100 -- -- This work

(R’ = CH(SiMe3)2, µ-H) Ph 100 -- --

SiMe3 -- 100 --

Cp*2ScMe H -- -- 100 1e

Me 100 -- --

Cp*2YCH(SiMe3)2 CMe3 100 0 -- 1k

Ph 89 11 --

SiMe3 20 80 --

Cp*2LaCH(SiMe3)2 Me 78 -- 21 1k

CMe3 100 -- --

Ph -- 86 14

SiMe3 4 45 51

Cp*2CeCH(SiMe3)2 Me 74 -- 25 1k

CMe3 100 -- --

Ph -- 82 18

SiMe3 7 61 32

Cp*Ce[CH(SiMe3)2]2 CMe3 100 -- -- 30

Cp*[ArO]YCH(SiMe3)2 SiMe3 -- 100 -- 4d

1,4-substituted 1-buten-3-yne C C

HR

H CC

R

2,4-substituted 1-buten-3-yne C C

RH

H CC

R

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

55

The regio- and stereoselectivity of the reaction depends strongly on the acetylene

substituent and catalyst applied. For Cp*2LnCH(SiMe3)2, the only other system for which the

dimerization of a variety of 1-alkynes has been investigated, only HC≡CCMe3 is selectively

coupled to the head-to-tail trans -H2C=C(CMe3)C≡CCMe3. For all other 1-alkynes tested,

HC≡CR (R = Me, Ph, SiMe3), a dramatic drop in selectivity was observed yielding either

mixtures of head-to-tail and head-to-head isomers (Ln = Y, La, Ce) or the additional formation

of higher oligomers (Ln = La, Ce, Table II). In our system, HC≡CMe, HC≡CPh and

HC≡CCMe3 are selectively coupled to head-to-tail dimers: 2,4-disubstituted trans -1-buten-3-

ynes (Scheme 2). However, for HC≡CSiMe3 a dramatic change in regioselectivity takes place,

exclusively yielding the head-to-head coupled product: trans -Me3SiC(H)=C(H)-C≡CSiMe3

(Scheme 2). This remarkable change in regioselectivity was also observed by Schaverien and

Heeres who found that Cp*[ArO]YCH(SiMe3)2 and Cp*2YCH(SiMe3)2 preferentially couple

HC≡CSiMe3 to trans -Me3SiC(H)=C(H)-C≡CSiMe3 (Table II). 1k,4b,28b The origin of this

change in regioselectivity is thought to be electronic (vide infra ).28b,31 That other factors also

play a decisive role is clearly illustrated by the [Cp*2ZrMe(B(4-C6H4F)4)] catalyzed coupling of

HC≡CSiMe3, which exclusively yields head-to-tail coupled dimers and trimers.18

Proposed Mechanism for the Dimerization of 1-Alkynes. The generally accepted

mechanism for the dimerization of 1-alkynes, catalyzed by group 3 metals and lanthanides is

plausible for our system as well.1d,e,k, 4b,20b,28b This mechanism consists of two interfering

processes: dissociation of the precursor into the catalytically active, unsolvated, monomeric

acetylides and the sequence of insertion and C-H bond activation reactions (Scheme 2). Den

Haan et al. 1d and Heeres et al. 1k proposed that the unsolvated monomeric acetylides are the

active species. These monomers leave sufficient space for an incoming alkyne, whereas the

free coordination site of the corresponding oligomeric compounds is blocked by bridging

ligands.1g,2,3e,4c,d If this assumption is correct, the catalytic activity should strongly depend on

the tendency of the dimeric acetylides to dissociate, resulting in catalytically active species.

Dissociation of the dimeric precursors is expected to be both sterically and electronically

controlled. When the steric strain within the dimeric acetylide complexes increases, the

dimeric structures become less favorable which will result in a higher concentration of

monomer (larger Keq, Scheme 2) and consequently in a higher observed catalytic activity. In

addition, the tendency to form dimeric species increases with increasing electrophilicity of the

metal center. Assuming that the electronic properties of {Cp2Y(µ-C≡CCMe3)}2 and {Cp*2Ln(µ-

Chapter 3.

56

C≡CCMe3)}n are comparable, the observation that {Cp2Y(µ-C≡CCMe3)}2 is inactive for the

dimerization of HC≡CCMe3,32 while the sterically more crowded {Cp*2Ln(µ-C≡CCMe3)}n is

very active in dimerizing HC≡CCMe3, suggests that the tendency to form inactive dimers is

mainly sterically controlled. The yttrium center in the bis(N,N’-bis(trimethylsilyl)benzamidinato)

system is more electrophilic than in the bis(pentamethylcyclopentadienyl) coordination

environment. Furthermore, the latter system is slightly more bulky.33 Hence, on the basis of

both steric and electronic arguments the gross catalytic activity of {[C6H5C(NSiMe3)2]2Y(µ-

C≡CR)}2 (3.1 - 3.6) in alkyne dimerization is expected to be lower than for the corresponding

{Cp*2Y(µ-C≡CR)}n compounds.

Scheme 2.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

57

R = Ph, CMe3

HC CR

THF

R = SiMe3

Y

N

N

N THF

N CCR

Y

N

N

NN

CC

Y

N

N

NN

CR

CR

Y

N

N

N

N

H

SiMe3

SiMe3

CY

N

N

N THF

N CH2Ph

Keq. Keq.

HC CR

Y

N

N

N

NC CR

H

R

R

NN

N

N

YSiMe3

H

Me3Si

NN

N

N

Y

HC CR

HC CRHC CR

C

C C

H

HR

C

R

C

CC

H

R H

C

R

The observed catalytic activity of the complexes 3.4 - 3.6 to dimerize the corresponding 1-

alkynes (HC≡CR, R = SiMe3, Ph, CMe3 respectively), is indeed much lower than found for the

{Cp*2Y(µ-C≡CR)}n analogues, while the sterically less demanding acetylides 3.1 - 3.3 are

even inactive for the dimerization of the corresponding alkynes (HC≡CR, R = H, Me, n-Pr

respectively). This observation supports the earlier made assumption that the corresponding

monomeric acetylides are the active species.

Chapter 3.

58

During the dimerization of HC≡CR (R = SiMe3, Ph, CMe3), notable amounts of the dimeric

acetylides (3.4 - 3.6 respectively) are present, while no monomeric yttrium acetylides could be

detected (1H NMR spectroscopy). This indicates that for 3.4 - 3.6 Keq is small. Assuming that

less than 3 % of the catalyst is active (detectability limit for 1H NMR ≈ 3 %), a minimum Nt

(80oC) of 1300 mol.mol-1.h-1 can be estimated, which is in good agreement with the observed

Nt (25oC) of ≥ 1900 mol.mol-1.h-1 for 1-alkyne dimerization catalyzed by {Cp*2Ln(µ-

C≡CCMe3)}n. Hence, the intrinsic catalytic activity of the bis(benzamidinato) yttrium acetylides

is similar to that observed for the corresponding {Cp*2Ln(µ-C≡CR)}n complexes (vide supra ).

The cause of the low tendency of the dimeric acetylides to dissociate cannot simply be

assigned to either steric or electronic effects, but is probably a combination of both. Since the

steric bulk of the bis(benzamidinato) ligand system is slightly less than of the bis(pentamethyl-

cyclopentadienyl) ligand set, it is not very convincing to attribute the differences in reactivity to

steric reasons alone. On the other hand, the fact that bulky 1-alkynes are selectively dimerized

while for small 1-alkynes the reaction stops on formation of the dimeric acetylide complexes

proves that electronic effects are not the only reason either. If this was the case, the rate of

dimerization should be the same for both small and bulky 1-alkynes, which is clearly not true.

The same arguments can be used to explain the low turnover frequency observed when the

THF adduct 3.8 is used as precursor. Like dissociation of the dimeric acetylides, the tendency

of 3.8 to release the coordinated THF is very low.

During the insertion step, the regioselectivity is determined. On the basis of steric

considerations, the head-to-tail dimer is expected, as found for the dimerization of HC≡CR

(R= Me, Ph, CMe3; Scheme 2). To form the head-to-head product, as observed for

HC≡CSiMe3, the acetylene substituent has to point in the direction of the bulky ancillary ligand

system during insertion (Scheme 2). Sterically this will be unfavorable. Therefore, the

exclusive formation of trans -Me3SiC(H)=C(H)-C≡CSiMe3 has to be electronically and not

sterically controlled.4d,28b,31 To complete the catalytic cycle, C-H bond activation of the

insertion product results in release of the 1-buten-3-yne and formation of the monomeric

acetylide (Scheme 2). Insertion, instead of C-H bond activation, of another alkyne into the

yttrium-1-buten-3-ynyl species is clearly unfavorable, since higher oligomers (GC-MS) were

not found.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

59

Time (min).

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

326.0 K

332.6 K

338.6 K

344.3 K

350.4 K

Figure 4. Normalized ratio of substrate to yttrium concentration as a function of time and

temperature for the dimerization of HC≡CCMe3 using [C6H5C(NSiMe3)2]2YCH(SiMe3)2 as

precatalyst.

One question that remains is which step of this process is rate-determining. Kinetic

experiments carried out for the dimerization of HC≡CSiMe3 and HC≡CCMe3 showed clean

frist order dependence in 1-alkyne (Figure 4). The fact that the amount of the dimeric

acetylides (3.4 - 3.6) remains constant during the dimerization reactions, suggests that

insertion of an incoming acetylene into the Y-C≡CR bond is rate-determining, with the

assumption that equilibration (Keq, Scheme 2) is rapid. If C-H bond activation was rate-

limiting, build-up of the intermediates [Y]-C(H)=C(CMe3)-C≡CCMe3 or [Y]-C(SiMe3)=C(H)-

C≡CSiMe3 would be expected. However, since Keq is very small, only a very small amount of

active catalyst is present determining the rate-determining is tricky.

Apparently, the same process that reduces the catalytic activity of the bis(N,N’-

bis(trimethylsilyl)benzamidinato) yttrium compounds in olefin polymerization (slow dissociation

of the dimeric precursors into the active catalyst) is also responsible for the low activity in the

catalytic dimerization of 1-alkynes. Nevertheless, these results show that the intrinsic catalytic

activity of yttrium bis(benzamidinato) compounds in 1-alkyne dimerization is very similar to

that observed for the analogous Cp*2YR systems.

3-6. Reactivity Towards Nitriles.

Chapter 3.

60

The reactivity of group 3 and 4 metals and lanthanides is dominated by their propensity to

form strong bonds with hard donor atoms such as oxygen and nitrogen. Therefore, it has

generally been assumed that early transition metal complexes containing these bonds are too

inert to function as catalysts. Recently however, several examples appeared in the literature

reporting reactive M-O and M-N bonds, which illustrates that amido and oxo compounds can

function as catalysts or stoichiometric reagents.1o,q,s, 3d,20b,34 Therefore, we argued that

oligo/polymerization and catalytic hydrogenation of nitriles may be possible. The reaction of

[C6H5C(NSiMe3)2]2YR (R = CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µ-H (2.9)) with N≡CCMe3

and N≡CMe has been studied in more detail and the results are presented in this section.

Insertion Versus C-H Bond Activation. An NMR tube reaction showed that N≡CCMe3 (1

equiv., benzene-d6) does not react or even coordinate with 2.8. This is against expectation,

since the sterically more hindered Cp*2YCH(SiMe3)2 reacts smoothly with N≡CCMe3 to yield

the imido, Cp*2Y(N=C(CMe3)CH(SiMe3)2).N≡CCMe3. When 2.8 is treated with N≡CMe, C-H

bond activation rather than insertion takes place, yielding H2C(SiMe3)2 and the novel dimeric

crotonitrileamido complex, {[C6H5C(NSiMe3)2]2Y(µ-(N,N’)-N(H)-C(Me)=C(H)-C≡N)}2 (3.10,

eq. 6). This contrasts with the reactions of {[C5H4R]2Y(µ-H).THF}2 (R = H, Me) and Cp*2ScR

(R = p-Me-C6H4, Me) with acetonitrile: both resulted in clean insertion.1c,34a Metalation was

also observed for Cp*2LnCH(SiMe3)2 (Ln = La, Ce) with acetonitrile, affording dimeric

cyanomethyl complexes, {Cp*2Ln(CH2C≡N)}2.1i For our system, this compound could not be

detected. However, it is likely that it is formed as an intermediate which reacts further by

insertion of another equivalent of acetonitrile, subsequently followed by a 1,3-H shift, to give

3.10 (eq. 6). The reaction compares well with the alkali metal catalyzed dimerization of

acetonitrile to crotonitrileamides,35,36 and is analogous to the reaction of the corresponding

bis(N,O-bis(tert -butyl)alkoxysilylamido) yttrium bis(trimethylsilyl)methyl compound,

[Me2Si(NCMe3)(OCMe3)]2YCH(SiMe3)2 (4.6), with N≡CMe. In the latter case,

{[Me2Si(NCMe3)(OCMe3)]2Y(µ-(N,N’)-N(H)-C(Me)=C(H)-C≡N)}2 (4.8) was isolated and

structurally characterized (Chapter 4). The IR and NMR data for 3.10 are in good agreement

with those of Na[N(H)-C(Me)=C(H)-C≡N]35,36 and {[Me2Si(NCMe3)(OCMe3)]2Y(µ-(N,N’)-

N(H)-C(Me)=C(H)-C≡N)}2 (4.8, Chapter 4). The lowfield shift of the C(Me)=C (δ = 131.3 ppm)

and the highfield shifts of the C(H)=C (δ = 48.1 ppm) and C(H)=C (δ = 3.50 ppm) resonances

in the 13C and 1H NMR spectra of 3.10 clearly demonstrate the extensive charge

delocalization within the crotonitrileamido fragment.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

61

N CMe

C-H activation

Insertion

1,3-H shift

N CMeY

N

N

N

NC

SiMe3

SiMe3

H

C

N:

CH2

Me

CN

C

:N

CH2 Me

CN

Y

N

N

NN Y

N

N

NN

C C

Me

NC

H

HN

CC

Me

N C

H

H N

Y

N

N

N

N

Y

N

N

N

N

CH2

CN

NC

CH2

Y

N

N

N

N

Y

N

N

N

N

(6)

2.8

3.10

It appears that the bulky carbyl group in 2.8 acts as a classical Brønsted base, while the

sterically less hindered N≡CCH2 fragment of the intermediate cyanomethyl complex is a better

nucleophile which leads to insertion of another acetonitrile. When the reaction of 2.8 with

exactly one equivalent of acetonitrile was carried out, a 1 : 1 mixture of 2.8 and 3.10 was

obtained. Hence, the insertion of the second equivalent of N≡CMe is faster than the preceding

metalation step. In contrast, for Cp*2LnCH(SiMe3)2 (Ln = La, Ce) C-H bond activation is faster

as is illustrated by the selective formation of the cyanomethyl complex.

Nitrile insertion was observed for 2.7 with N≡CCMe3, initially yielding

{[C6H5C(NSiMe3)2]2Y(µ-N=C(CMe3)CH2Ph)}2 (3.11a), which completely rearranges to

{[C6H5C(NSiMe3)2]2Y(µ-NH-C(CMe3)=C(H)Ph)}2 (3.11b, eq. 7) as a result of imine-enamine

tautomerism. In a similar manner, 2.7 reacts with acetonitrile but here a mixture of compounds

is obtained. Besides the formation of the imido {[C6H5C(NSiMe3)2]2Y(µ-N=C(Me)CH2Ph)}2

(3.12a, 50 %) and enamido {[C6H5C(NSiMe3)2]2Y(µ-NH-C(Me)=C(H)Ph)}2 (3.12b, 40 %)

derivatives, competitive deprotonation of acetonitrile is observed yielding the crotonitrileamido

(3.10, 10 %, eq. 8).

Chapter 3.

62

Y

N

N

N THF

N CH2Ph insertion

N CCMe3

2.7

3.11a

Y

N

N

N

NY

N

N

N

N

CMe3

C

PhCH2

N

N

CH2Ph

C

Me3C

1,3-H shift (7)

3.11bC

Ph

H

N

CCMe3

H

CPh

H

N

CMe3C

HY

N

N

N

N Y

N

N

N

N

Remarkably is the influence of the Me versus CMe3 substituent on the N≡CR: In contrast to

the complete conversion of 3.11a into 3.11b, the imido 3.12a does not completely rearrange

into the enamido 3.12b, but remains present even after prolonged periods of time.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

63

Y

N

N

N THF

N CH2Ph+

2.7

C C

Me

NC

H

HN

CC

Me

N C

H

H N

Y

N

N

N

N

Y

N

N

N

N

N CMe

(8)+

C

Ph

H

N

CMe

H

CPh

H

N

CMe

HY

N

N

NN Y

N

N

NNY

N

N

N

NY

N

N

N

N

Me

C

PhCH2

N

N

CH2Ph

C

Me

(3.10, 10 %)

(3.12a, 50 %) (3.12b, 40 %)

Bis(N,N’-bis(trimethylsilyl)benzamidinato yttrium hydride (2.9) reacts instantaneously at

room temperature with nitriles, N≡CR (1 : 1 ratio; R = Me, CMe3, eq. 9).

2.9

(9)Y

N

N

NN

HH

Y

N

N

NN N CR

R

C

H

N

N

HC

R

Y

N

N

N

N Y

N

N

N

N

3.13, R = Me;3.14, R = CMe3.

Resonances characteristic for insertion products {[C6H5C(NSiMe3)2]2Y(µ-N=C(H)R)}2 (R =

Me (3.13), CMe3 (3.14)) were observed in the 1H NMR spectra, although the products are

unstable and decompose rapidly at room temperature. The 1H NMR spectra of both

compounds are consistent with those of the corresponding {[C5H4R]2Y(µ-N=C(H)R’)}2 (R = H,

R’ = Me, CMe3; R = Me, R’ = Me) species, prepared in similar fashion by Evans et al. 1b

Chapter 3.

64

These results show that the competition between nucleophilicity and Brønsted basicity of

the R-group in [C6H5C(NSiMe3)2]2YR (R = CH(SiMe3)2, CH2Ph, µ-H) has a strong influence

on the reaction pattern of these compounds with α-hydrogen containing nitriles. The bulky

bis(trimethylsilyl)methyl substituent tends to react as a classical Brønsted base such as M2-

naphthalene (M = Li, Na, K, MgCl)35 and NaN(SiMe3)2,36 deprotonating acetonitrile. With only

10 % deprotonation of acetonitrile, the benzyl group in 2.7 clearly shows a reduced Brønsted

base character, and resembles Cp*2ScR (R = p-Me-C6H4, Me),34a [Cp’2MR]+ (Cp’ = C5H5,

C5H4Me; M = Ti, Zr; R = alkyl)37a-c and Al(NEt2)Et2.38 As a result of its highly nucleophilic

character, the hydride 2.9 only shows insertion of nitriles, analogous to Cp*2ScH,34a

Cp*2ZrH2,34a and [Cp’2ZrH]+ (Cp’ = C5H5, C5H4Me).37c,d

Hydrogenation of Nitriles. The fact that the Ln-N (Ln = Sc, Y, lanthanides) bond is less

inert as previously assumed1o,s,3d,20b,34a is clearly emphasized by the Cp*2ScH catalyzed

hydrogenation of nitriles.34a To investigate the reactivity of the Y-N bond in our system, the

reaction of 2.9 with N≡CCMe3 in the presence of hydrogen (4 atm.) was studied.

Following the reaction by 1H NMR spectroscopy revealed that initially a mixture was

formed amongst which 3.14 could clearly be identified. After 24 hours at room temperature,

the hydrogenated product [C6H5C(NSiMe3)2]2Y-N=C(CMe3)N(H)CH2CMe3 (3.15) had been

formed in 90 % yield (eq. 10).39 This compound does not react further with dihydrogen or

N≡CCMe3. Even after days at 60oC, no free H2NCH2CMe3 had been formed. This is not

surprising, since formation of the corresponding scandium compound, Cp*2Sc(N=C(CMe3)-

N(H)CH2CMe3), is responsible for catalyst deactivation during the scandium catalyzed

hydrogenation of nitriles. The mechanism of the hydrogenation-dimerization reaction (eq. 10)

is assumed to be analogous to that observed for Cp*2ScH with N≡CCMe3 in the presence of

hydrogen and will not be further discussed.38

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

65

CMe3

C

H

N

N

H

C

Me3C

Y

N

N

N

N Y

N

N

N

NY

N

N

NN

HH

Y

N

N

NN

3.14

3.15

(10)2 Y

N

N

NN N

CN

CMe3

H2C CMe3

H

2.9

N CCMe3

H2

N CCMe3

3-7. Reactivity Towards Activated Arenes.

C-H bond activation or ortho -metalation of aromatic molecules C6H5X (X = H, Me, OMe,

SMe, NMe2, CH2NMe2, PMe2), P(=CH2)Ph3, mesitylene, pyridine and α-picoline is a

common process for Cp*2LnR (Ln = Sc, Y, lanthanides; R = alkyl, H) species; it provides a

very clean route towards new carbyl derivatives.1a,d,e, 40

As discussed above, heating of benzene or toluene solutions of 2.7 - 2.9 did not lead to

metalation or H/D scrambling. Furthermore, the stability of 2.8OMe and 2.9OMe and the lack of

reactivity of 2.9 towards C6H5X (X = OMe, CH2NMe2) demonstrates a much lower tendency

of the bis(benzamidinato) yttrium compounds to ortho -metalate compared with their Cp*2LnR

congeners (Chapter 2). This can be assigned to the different character of the Y-C or Y-H bond

which is a clear result of the different electronic properties of the benzamidinato ligands.

However, with pyridine or α-picoline fast reactions are observed.

C-H Bond Activation Versus Insertion. When [C6H5C(NSiMe3)2]2YCH(SiMe3)2 (2.8)

was treated with pyridine, a complicated reaction occurred. Quantitative formation of the

alkane, H2C(SiMe3)2, indicated metalation of pyridine. However, the expected pyridyl species

could not be detected by 1H NMR spectroscopy. Vinylic C-H resonances (1H NMR: δ = 4 - 6

Chapter 3.

66

ppm) suggest partial reduction of pyridine. It is probable that insertion of another equivalent of

pyridine into the Y-C bond of the initially formed pyridyl derivative occurred, similar as

proposed for the reaction of Cp*2Y(η2-(C,N)-2-NC5H4) with pyridine (eq. 11).40

Y

N

N

N

N

H

SiMe3

SiMe3

C

2.8

N

N

(11)

Y

N

N

N

NN

Y

N

N

N

N N

N H

While {Cp*2Y(µ-H)}2 is an excellent precursor for the pyridyl derivative, Cp*2Y(η2-(C,N)-2-

NC5H4),1d,40 reaction of {[C6H5C(NSiMe3)2]2Y(µ-H)}2 (2.9) with pyridine resulted in insertion

rather than C-H bond activation, as the 1,2-inserted product [C6H5C(NSiMe3)2]2Y-NC5H6

(3.16, eq. 12) was obtained. Hence, the reactivity of 2.9 towards pyridine, resembles that of

{[C5H4R]2Y(µ-H).THF}21b and the in situ prepared bis(alkoxysilylamido) yttrium hydride,

[Me2Si(NCMe3)(OCMe3)]2YH.Py (Chapter 5). Subsequent thermal isomerization to the 1,4-

addition product, as observed for the latter two compounds, did not occur in this system (days,

100oC).

3.16, R = H,3.17, R = CH2Ph.

(12)Y

N

N

N

N R

N

Y

N

N

N

N N

RH

2.9, R = H;2.7, R = CH2Ph.THF.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

67

Since the reactivity of the benzyl derivative, [C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) towards

acetonitrile was found to be intermediate between that of the bis(trimethylsilyl)methyl

compound 2.8 (C-H bond activation) and the hydride 2.9 (insertion), we were interested to see

how it would react with excess pyridine. Compound 2.7 reacts instantaneously with pyridine

(2.5 equiv.) by insertion to form exclusively the 1,2-insertion product, [C6H5C(NSiMe3)2]2Y-

NC5H5-2-CH2Ph (3.17, eq. 12). Unfortunately, its extreme solubility precluded purification,

although it could conclusively be identified (1H, 13C NMR, COSY, NOESY). Hence, the

reactivity of 2.7 towards pyridine is very similar to that of 2.9 and alkyllithium reagents. The

Ziegler alkylation of pyridines by alkyllithium reagents involves intermediate 1,2-insertion

products which subsequently undergo elimination of LiH.41 However, analogous formation of

the yttrium hydride {[C6H5C(NSiMe3)2]2Y(µ-H)}2 (2.9) and release of 2-benzylpyridine was not

observed in this system.

Y

N

N

N

N R

N

3.18

(13)Y

N

N

N

NN

CH2

2.7, R = CH2Ph.THF;2.8, R = CH(SiMe3)2;2.9, R = H.

Since its methyl protons are rather acidic, for 2.7 and 2.9 metalation of picoline was

expected to become competitive to insertion. NMR tube reactions of 2.7, 2.8 and 2.9 with α-

picoline indeed showed the formation of toluene, H2C(SiMe3)2 and H2 respectively, together

with the exclusive formation of the α-picolyl derivative, [C6H5C(NSiMe3)2]2Y(η2-(C,N)-CH2-2-

NC5H4) (3.18, eq. 13). For 2.7, the reaction is complete within hours at room temperature,

whereas for 2.8 and 2.9 heating to 100oC (hours) was required. While this reaction is strictly

analogous to that of 4.6 (Chapter 5) and alkyllithium reagents with α-picoline,42 it fully

contrasts that of the corresponding Cp*2YR (R = Me.THF, CH(SiMe3)2) systems, which

afforded the corresponding pyridyl derivative, Cp*2Y(η2-(C,N)-2-NC5H3-6-Me).1d This again

clearly demonstrates the difference in Y-C bond character as a result of electronic changes in

the ancillary ligand system. The NMR data of 3.18 are nearly identical to those of the

corresponding, structurally characterized, bis(alkoxysilylamido) yttrium α-picolyl species,

Chapter 3.

68

[Me2Si(NCMe3)(OCMe3)]2Y(η2-(C,N)-CH2-2-NC5H4) (5.2, Chapter 5), illustrating the high

similarity between both systems.

3-8. Concluding Remarks.

The results presented in this chapter clearly illustrate similarities and differences in

reactivity between [C6H5C(NSiMe3)2]2YR and the corresponding Cp*2YR (R = alkyl, H)

complexes as a result of the different character of the Y-R bond. Like Cp*2YR,

[C6H5C(NSiMe3)2]2YR is a catalyst for ethylene polymerization and 1-alkyne dimerization,

although for the bis(benzamidinato) yttrium complexes, reaction rates are much lower. This

seems to be a result of their high tendency to form stable, inactive dimers. The cause for this

behavior cannot simply be assigned to either steric or electronic effects, but is a combination

of both. Since the steric bulk of the bis(benzamidinato) ligand system is slightly less than that

of the bis(pentamethyl-cyclopentadienyl) ligand set, it is not very convincing to attribute the

differences in reactivity to steric reasons alone. On the other hand, the fact that bulky 1-

alkynes are selectively dimerized while for small 1-alkynes the reaction stops with the

formation of the dimeric acetylide complexes, shows that electronic effects are not the only

reason either. If the latter was true, the rate of dimerization should be the same for both small

and bulky 1-alkynes, which is clearly not the case.

A fundamental difference between the bis(benzamidinato) yttrium alkyl and hydrido

complexes and the corresponding (permethylated) yttrocene alkyl and hydrido compounds is

the Brønsted basic behavior of the former towards nitriles and (substituted) pyridines. Hence,

the reactivity of bis(benzamidinato) yttrium alkyl complexes resembles that of alkyllithium

reagents and suggests a highly polarized/ionic system. This is supported by the easy loss of

benzamidinato ligands to other Lewis acids as was observed during the reaction of

[C6H5C(NSiMe3)2]2YCl.THF with LiAlH4. This is characteristic for ionic systems (Chapter 2).

3-9. Experimental Section.

Material and Methods. All compounds are extremely oxygen- and moisture-sensitive.

Manipulations were therefore carried out under nitrogen by using glovebox (Braun MB-200) and

Schlenk techniques. Hydrogen (Hoek-Loos 99.9995 %), ethylene, propylene (DSM Research B.V.),

acetylene (Ucar, C.P.) and propyne (Matheson, C.P.) were used as purchased. 1-Hexene, alkynes,

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

69

nitriles, pyridine and picoline (Janssen) were dried over 4Å molecular sieves and distilled prior to

use. Benzene-d6 was degassed and dried over Na/K alloy. All solvents were distilled from Na

(toluene), K (THF) or Na/K alloy (ether, pentane) and stored under nitrogen. [C6H5C(NSiMe3)2]2YR

((µ-Me)2.TMEDA (2.6), CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µ-H (2.9) were prepared according to

procedures described in Chapter 2. Experimental details of the [C6H5C(NSiMe3)2]2YR (R =

CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µ-H (2.9)) catalyzed hydroboration of 1-hexene is described

elsewhere.14

Physical and Analytical Measurements. NMR spectra were recorded on a Varian VXR 300 (1H

NMR at 300 MHz, 13C NMR at 75.4 MHz) spectrometer. 1H and 13C NMR spectra were referenced

internally using the residual solvent resonances. IR spectra were recorded on a Mattson-4020

Galaxy FT-IR spectrophotometer. Elemental analyses were carried out at the Analytical Department

of this laboratory. Quoted data are the average of at least two independent determinations. The

polyethylene high temperature GPC was performed by Polymer Laboratories LTD. Polystyrene was

used as calibrant.

NMR Tube Reaction of [C6H5C(NSiMe3)2]2YR (R = CH2Ph.THF (2.7), µµ-H (2.9)) with Ethylene.

NMR tubes containing benzene-d6 (0.5 mL) solutions of 2.7 or 2.9 (0.01 - 0.02 mmol) were charged

with ethylene (4 atm) and monitored at fixed time intervals. Heating was required for the reaction to

proceed. At 65oC the ethylene was gradually consumed with formation of polyethylene. With 2.7, the

ethylene was consumed after several hours. For 2.9 the reaction is complete within 30 minutes (1H

NMR). The contents of the NMR tubes were quenched with methanol. The polyethylene was filtered

off, dried and characterized by IR.

Ethylene Polymerization Experiment. A 200 mL autoclave was charged with 100 mg (0.16

mmol) of 2.9 dissolved in 75 mL toluene. The autoclave was closed, pressurized with ethylene (70

atm) and heated to 50oC. After 1.5 hour, the autoclave was cooled to room temperature and the

pressure released. After quenching the reaction mixture (methanol), the polyethylene was filtered and

washed with methanol and pentane. After drying, 1.0 g polyethylene was obtained. The product was

characterized by IR spectroscopy and high temperature GPC: Mn = 46300, Mw = 239900, Mw/Mn =

5.2.

Preparation of {[C6H5C(NSiMe3)2]2Y(µµ-C≡≡CR)}2 (3.1, R = H; 3.2, R = Me). 3.1: (a) A 100 mL

Schlenk flask charged with a solution of 2.9 (1.4 g, 1.1 mmol) in benzene (10 mL) was treated with

acetylene (1.5 mmol) and allowed to stand for three days at room temperature, upon which large

colorless crystals of 3.1 were formed (1.1 g, 0.86 mmol, 78%). (b) A 100 mL Schlenk flask charged

with a solution of 2.8 (1.95 g, 2.52 mmol) benzene (10 mL) was filled with acetylene (2.6 mmol). The

flask was allowed to stand for three days at room temperature, upon which large colorless crystals of

3.1 were formed (1.3 g, 1.0 mmol, 80%). IR (KBr/Nujol, cm-1): 3262(w), 2926(vs), 2855(s), 1914(w),

1445(vs), 1387(vs), 1242(s), 1003(m), 982(s), 841(vs), 785(w), 760(s), 725(w), 700(w), 679(w),

482(w). 1H NMR (benzene-d6, δ): 7.33 (m, 4H, Ar), 7.10 (m, 6H, Ar), 3.22 (s, 1H, C≡CH), 0.19 (s,

Chapter 3.

70

36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 184.5 (s, C6H5C(NSiMe3)2), 146.7 (dt, Y-

C(≡CH)-Y, 1JY-C = 20 Hz, 2JC-H = 27 Hz), 143.2 (s, Ar), 128.1 (d, Ar, 1JC-H = 218 Hz), 126.5 (d, Ar,1JC-H = 160 Hz), 115.4 (d, C≡CH, 1JC-H = 218 Hz), 3.19 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz).

Anal. Calcd. (found) for C56H94N8Si8Y2: C: 52.47 (52.83); H: 7.39 (7.52); Y: 13.87 (13.91). 3.2:

Compound 3.2 was prepared by a similar procedure from 2.8 (1.00 g, 1.29 mmol) and propyne; it was

isolated as microcrystalline material (0.60 g, 0.46 mmol, 71 %). IR (KBr/Nujol, cm-1): 2924(vs),

2855(s), 2060(w), 1445(vs), 1429(vs), 1402(vs), 1246(s), 1005(m), 982(s), 843(vs), 785(w), 760(s),

727(w), 702(w), 480(w). 1H NMR (benzene-d6, δ): 7.37 (m, 4H, Ar), 7.10 (m, 6H, Ar), 2.01 (s, 3H,

C≡CCH3), 0.21 (s, 36H, C6H5C(NSi(CH3)3)2). 13C{1H} NMR (benzene-d6, δ): 184.8 (s,

C6H5C(NSiMe3)2), 143.3 (s, Ar), 136.0 (t, Y-C(≡CMe)-Y, 1JY-C = 21 Hz), 128.1 (s, Ar), 126.8 (s, Ar),

8.0 (s, C≡CCH3), 3.9 (s, C6H5C(NSi(CH3)3)2). Anal. Calcd.(found) for C58H98N8Si8Y2: C: 53.18

(53.85); H: 7.54 (7.56); Y: 13.57 (13.48).

NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with 1-Pentyne. To an NMR tube

charged with a benzene-d6 (0.7 mL) solution of 2.9 (32 mg, 0.052 mmol), 1-pentyne (5.5 µL, 0.056

mmol) was added. After 24 hours at room temperature, all 1-pentyne had been consumed to give

{(C6H5C(NSiMe3)2)2Y(µ-C≡C-n-Pr}2 (3.3) as the only product. 1H NMR (benzene-d6, δ): 7.40 (m, 4H,

Ar), 7.07 (m, 6H, Ar), 2.60 (t, 2H, C≡CCH2), 1.91 (dt, 2H, CH2CH2CH3), 1.03 (t, 3H, CH2CH3), 0.26

(s, 36H, C6H5(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 185.1 (s, C6H5C(NSiMe3)2), 143.3 (s, Ar),

136.5 (t, Y-C(≡C(CH2)2CH3)Y, 1JY-C = 21 Hz), 132.4 (s, Y-C≡C), 128.4 (d, Ar, 1JC-H = 155 Hz), 127.7

(d, Ar, 1JC-H = 159 Hz), 126.5 (d, Ar, 1JC-H = 173 Hz), 25.1 (t, C≡CCH2CH2, 1JC-H = 129 Hz), 22.8

(m, CH2CH2CH3), 14.4 (q, CH2CH3, 1JC-H = 117 Hz), 4.1 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 117 Hz).

Preparation of {[C6H5C(NSiMe3)2]2Y(µµ-C≡≡CR)}2 (3.4: R = SiMe3; 3.5: R = Ph; 3.6: R = CMe3).

3.4: At room temperature, HC≡CSiMe3 (0.34 mL, 2.47 mmol) was slowly diffused into a pentane (5

mL) solution of 2.8 (1.65 g, 2.13 mmol). After 24 h, the colorless microcrystalline material formed

was washed with pentane (5 mL) and dried in vacuo , yielding 1.00 g (1.4 mmol, 66%) of 3.4. IR

(KBr/Nujol, cm-1): 2957(vs), 2926(vs), 2855(s), 1983(w), 1952(w), 1904(w), 1447(vs), 1435(vs),

1389(vs), 1246(s), 1005(m), 986(s), 922(w), 843(vs), 783(w), 760(s), 729(w), 702(w), 677(w),

650(w), 567(w), 480(w), 438(w). 1H NMR (benzene-d6, δ): 7.49 (m, 4H, Ar), 7.10 (m, 6H, Ar), 0.54 (s,

9H, C≡CSi(CH3)3), 0.28 (s, 36H, C6H5C(NSi(CH3)3)2). 13C{1H} NMR (benzene-d6, δ): 185.6 (s,

C6H5C(NSiMe3)2), 172.3 (t, Y-C(≡CSiMe3)-Y, 1JY-C = 20 Hz) 143.1 (s, Ar), 140.3 (s, C≡CSiMe3),

128.3 (s, Ar), 127.9 (s, Ar), 125.6 (s, Ar), 4.8 (s, C6H5C(NSi(CH3)3)2), 2.8 (s, C≡CSi(CH3)3). Anal.

Calcd.(found) for C62H110N8Si10Y2: C: 52.21 (52.35); H: 7.77 (7.81); Y, 12.47 (12.57).

Compounds 3.5 and 3.6 were prepared by a similar procedure and isolated as colorless crystals in

70% (3.5) and 77% (3.6) yield. 3.5: NMR data and elemental analysis showed that 3.5 contained 0.25

equivalent pentane per dimer in the crystal lattice. IR (KBr/Nujol, cm-1): 3040(w), 2912(vs), 2857(vs),

2049(m), 1883(w), 1769(w), 1597(w), 1576(w), 1441(vs), 1414(vs), 1381(vs), 1072(m), 982(m),

843(vs), 785(s), 756(vs), 721(vs), 702(s), 689(s), 544(m), 482(s), 438(m). 1H NMR (benzene-d6, δ):

7.92 (d, 2H, Ar), 7.48 (m, 4H, Ar), 7.07(m, 9H, Ar), 1.23 (m, 1.5H, pentane), 0.88 (m, 1.5H, pentane),

0.16 (s, 36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 184.8 (s, C6H5C(NSiMe3)2, 143.0 (s,

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

71

Ar), 141.8 (t, Y-C(≡CPh)-Y, 1JY-C = 21 Hz), 133.7 (dt, Ar, 1JC-H = 162 Hz, 2JC-H = 7 Hz), 131.1 (s,

C≡CPh), 129.3 (dt, Ar, 1JC-H = 158 Hz, 2JC-H 7 Hz), 124.6 (t, Ar, 2JC-H = 9 Hz), 22.7 (t, pentane, 1JC-H

= 124 Hz), 14.2 (q, pentane, 1JC-H = 124 Hz), 4.0 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 119 Hz). Anal.

Calcd.(found) for C68H112N8Si8Y2(C5H12)0.25 : C: 57.60 (57.44); H: 7.40 (43); Y: 12.09 (12.25). 3.6: IR

(KBr/Nujol, cm-1): 3057(w), 2957(vs), 2926(vs), 2870(s), 2855(s), 2037(m), 1443(s), 1429(vs),

1414(vs), 1397(vs), 1248(vs), 1003(w), 980(s), 839(vs), 785(m), 758(s), 748(s), 721(s), 704(s),

480(s), 438(m), 415(m). 1H NMR (benzene-d6, δ): 7.39 (4H, Ar), 7.04 (m, 6H, Ar), 1.52 (s, 9H,

C(CH3)3), 0.19 (s, 36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 185.7 (s, C6H5C(NSiMe3)2),

143.1 (s, Ar), 141.8 (t, Y-C(≡C(CMe3))-Y, 1JY-C = 22 Hz), 140.2 (s, C≡C(CMe3)), 128.2 (dt, Ar, 1JC-H

= 160 Hz, 1JC-H = 7 Hz), 127.4 (dd, Ar, 1JC-H = 117 Hz, 1JC-H = 7 Hz), 31.8 (q, C(CH3)3, 1JC-H = 126

Hz), 4.8 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 119 Hz). Anal. Calcd.(found) for C64H110N8Si8Y2: C: 55.14

(55.23); H: 7.95 (8.01); Y: 12.75 (12.82).

NMR Tube Reaction of 3.1 with THF; Preparation of [C6H5C(NSiMe3)2]2YC≡≡CH.THF (3.7). THF

(9 µL, 0.1 mmol) was added to an NMR tube charged with 3.1 (15 mg, 0.023 mmol) in benzene-d6

(0.5 mL). The 1H NMR collected after 5 minutes at room temperature showed the quantitative

formation of 3.7 and the presence of free THF. 1H NMR (benzene-d6, δ): 7.25 (m, 4H, Ar), 7.07 (m,

6H, Ar), 3.70 (m, 17.2 H, THF-α-CH2), 1.99 (d, 1H, C≡CH, 3JY-H = 1.3 Hz), 1.45 (m, 17.2H, THF-β-

CH2), 0.16 (s, 36H, C6H5C(NSi(CH3)3)2). 13C{1H} NMR (benzene-d6, δ): 183.5 (s, C6H5C(NSiMe3)2),

143.4 (s, Ar), 128.2 (s, Ar), 128.0 (s, Ar), 126.2 (s, Ar), 89.2 (d, C≡CH, 1JY-C = 12 Hz), 68.4 (s, THF-

α-CH2), 25.6 (s, THF-β-CH2), 2.5 (s, C6H5C(NSi(CH3)3)2.

Preparation of [C6H5C(NSiMe3)2]2YC≡≡CCMe3.THF (3.8). At room temperature, THF (50 µL, 0.62

mmol) was added to a pentane (20 mL) solution of 3.5 (0.8 g, 0.57 mmol). After 10 minutes, the

solution was concentrated until crystallization started. Cooling to -30oC yielded 3.8 (0.63 g, 0.82

mmol, 72 %) as a white microcrystalline solid. IR (KBr/Nujol, cm-1): 3061(w), 2955(vs), 2922(vs),

2854(s), 2043(w), 2022(w), 1446(vs), 1431(vs), 1410(vs), 1366(s), 1244(s), 1005(m), 984(s), 916(w),

841(vs), 787(m), 758(s), 723(m), 700(m), 480(m), 457(w). 1H NMR (benzene-d6, δ): 7.17 (4H, Ar),

7.02 (m, 6H, Ar), 4.14 (m, 4H, THF-α-CH2), 1.44 (m, 4H, THF-β-CH2) 1.41 (s, 9H, C(CH3)3), 0.13 (s,

36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 182.8 (s, C6H5C(NSiMe3)2), 143.7 (s, Ar),

143.1 (s, Y-C(≡CCMe3)), 127.9 (dt, Ar, 1JC-H = 160 Hz, 2JC-H = 7 Hz), 127.8 (dd, Ar, 1JC-H = 160 Hz,2JC-H = 7 Hz), 126.2 (dt, Ar, 1JC-H = 160 Hz; 2JC-H = 7 Hz), 111.1 (d, Y-C(≡CCMe3), 1JY-C = 11 Hz),

71.1 (t, THF-α-CH2, 1JC-H = 150 Hz), 32.7 (q, C(CH3)3, 1JC-H = 126 Hz), 28.1 (s, CMe3), 25.3 (t,

THF-β-CH2, 1JC-H = 133 Hz), 2.56 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz). Anal. Calcd. (found) for

C36H63N4OSi4Y: C: 56.22 (56.31); H: 8.26 (8.34), Y: 11.56 (11.65).

Preparation of [C6H5C(NSiMe3)2]2Y(µµ-C≡≡CCMe3)2Li.TMEDA.(hexane) (3.9). To a solution of 2.6

(1.4 g, 1.8 mmol) in ether (40 mL), 3,3-dimethyl-1-butyne (0.45 mL, 3.7 mmol) was added. After 16

hours at room temperature, the solution was concentrated until crystallization started and cooled to -

30oC. Yield: 1.4 g (1.4 mmol, 75 %) of 3.9 as large colorless crystals. NMR data showed that 3.9

contains one equivalent of hexane in the crystal lattice. IR (KBr/Nujol, cm-1): 2956(vs), 2934(vs),

Chapter 3.

72

2826(vs), 2047(m), 1460(vs), 1454(vs), 1379(s), 1290(m), 1242(s), 1036(w), 1022(w), 1005(m),

984(s), 949(m), 847(s, br)785(s), 756(s), 735(s), 702(s), 683(m), 602(m), 478(m), 421(m). 1H NMR

(benzene-d6, δ): 7.42 (m, 4H, Ar), 7.14 (m, 6H, Ar), 2.33 (s, 12H, TMEDA-CH3), 2.02 (s, 4H,

TMEDA-CH2), 1.40 (s, 18H, C(CH3)3), 0.87 (m broad, 14H, hexane), 0.26 (s, 36H,

C6H5C(NSi(CH3)3)2. 13C NMR (benzene-d6, δ): 182.3 (s, C6H5C(NSiMe3)2), 144.2 (s, Ar), 127.8 (dd,

Ar, 1JC-H = 162 Hz, 2JC-H = 6Hz), 127.4 (d, Ar, 1JC-H = 160 Hz), 126.6 (d, Ar, 1JC-H = 166 Hz), 121.1

(d, Y-C, 1JY-C = 15 Hz), 57.1 (t, TMEDA-CH2, 1JC-H = 134 Hz), 46.9 (q, TMEDA-CH3, 1JC-H = 134

Hz), 32.4 (q, C(CH3)3, 1JC-H = 126 Hz), 3.3 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz). The hexane

from the crystal lattice could be removed by heating the compound in vacuo at 50oC. Anal. Calcd.

(found) for C44H85LiN6Si4Y: C: 58.31 (58.39); H: 9.45 (9.51); Y: 9.81 (9.92).

Catalytic Dimerization of Alkynes HC≡≡CR (R = Ph, CMe3, SiMe3) by [C6H5C(NSiMe3)2]2YR (R

= CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µµ-H (2.9), µµ-C≡≡CR’ (R’ = SiMe3 (3.4), Ph (3.5), CMe3 (3.6)).

Alkynes (100 µL, 0.9 - 0.7 mmol) were added to NMR tubes charged with benzene-d6 solutions (0.5

mL) of 2.7 - 2.9 (0.02 (± 5) mmol; catalyst to substrate ratios ranging from 1 : 35 to 1 : 60). The 1H

NMR spectra collected after 1 hour at 80oC indicated the quantitative formation of the 1-buten-3-

ynes, traces of unreacted 1-alkyne and characteristic resonances of {[C6H5C(NSiMe3)2]2Y(µ-

C≡CR)}2 (3.4 - 3.6), when 2.8, 2.9 or 3.4 - 3.6 were used as precursors. Nt(80oC) = 20 - 40 mol.mol-

1. h-1. For the dimerization of HC≡CMe3, catalyzed by 2.7 or 3.8, the 1H NMR spectra collected after

1 hour at 80oC showed the presence of H2C=C(CMe3)-C≡CCMe3, resonances attributable to 3.8 and

considerable amounts of unreacted HC≡CCMe3 (Nt (80oC) ≈ 1 mol. mol-1 .h-1 ).

NMR Kinetic Experiments for the Catalytic Dimerization of HC≡≡CCMe3 and HC≡≡CSiMe3. In a

typical kinetics experiment, an NMR tube was charged with a standard solution of 2.8 (0.17 mM) in

benzene-d6 (0.7 mL) and 1-alkyne (100 µL, 0.7-0.8 mmol). After the NMR tube was filled and sealed,

an array of 1H NMR spectra was recorded at constant temperature (± 0.2oC) and appropriate time

intervals until the reaction was completed. The kinetics were monitored by following the difference in

integral of the vinylic-protons of the 1-butene-3-ynes formed and the 1-alkyne acetylenic-proton. The

measurements were repeated at different temperatures ranging from 53-77oC.

Preparation of {[C6H5C(NSiMe3)2]2Y2(µµ-(N,N’)-N(H)-C(Me)=C(H)-C≡≡N)}2 (3.10). To a solution of

2.8 (1.5 g, 1.9 mmol) in pentane (15 mL), acetonitrile (200 µL, 3.8 mmol) was added at room

temperature. After stirring for 16 hours at room temperature, the formed yellow solution was

concentrated until crystallization started and the mixture was cooled to -30oC. After isolation (1.0 g),

the crude product was recrystallized from pentane (10 mL). Cooling to -30oC yielded 3.10 (0.8 g, 1.1

mmol, 60 %) as large colorless crystals. IR (KBr/Nujol, cm-1): 2953(s), 2924(vs), 2854(s), 2152(s),

1523(s), 1429(vs), 1313(m), 1246(s), 1180(m), 1074(w), 1030(w), 1004(m), 986(s), 920(m), 839(vs),

785(m), 760(s), 731(m), 702(s), 684)m), 638(w), 569(w), 482(m), 414(w). 1H NMR (benzene-d6, δ):

7.22 (m, 4H, Ar), 7.03 (m, 6H, Ar), 6.84 (s, 1H, NH), 3.50 (s, 1H, CH), 2.16 (s, 3H, CH3), 0.16 (s,

36H, C6H5C(NSi(CH3)3)2. 13C NMR (benzene-d6, δ): 183.6 (s, C6H5C(NSiMe3)2), 179.1 (s, C≡N),

143.2 (s, Ar), 131.3 (d, C(CH3)=C, 2JY-C = 5 Hz), 128.1 (d, Ar, 1JC-H = 161 Hz), 126.3 (d, Ar, 1JC-H =

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

73

159 Hz), 48.1 (d, CH, 1JC-H = 175 Hz), 25.1 (qd, CH3, 1JC-H = 127 Hz, 3JY-C = 10 Hz), 2.5 (q,

C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz). Anal. Calcd. (found) for C60H102N12Si8Y2: C: 51.70 (51.64), H:

7.38 (7.44), Y: 12.75 (12.78).

NMR Tube Reaction of [C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) with N≡≡CCMe3. An NMR tube was

charged with a solution of 2.7 (23 mg, 0.030 mmol) in benzene-d6 (0.5 mL). Then N≡CCMe3 (2.8 µL,

0.30 mmol) was added. The 1H NMR spectrum of the reaction mixture monitored after 10 minutes at

room temperature showed that all 2.7 was consumed and that {[C6H5C(NSiMe3)2]2Y(µ-

N=C(CMe3)CH2Ph)}2 (3.11a) was formed. Within hours at room temperature, compound 3.11a

gradually rearranged into the enamine isomer {[C6H5C(NSiMe3)2]2Y(µ-NH-C(CMe3)=C(H)Ph)}2(3.11b). 1H NMR (benzene-d6, δ) of 3.11a: 7.55 (d, 2H, Ar, 3JH-H = 10 Hz), 7.30 (m, 6H, Ar), 7.16 (m,

7H, Ar), 3.93 (2H, CH2), 1.37 (s, 9H, C(CH3)3), 0.14 (s, 36H, C6H5C(NSi(CH3)3)2). The 1H NMR

spectrum indicated that the THF was released. For NMR data of 3.11b see below.

Preparation of {[C6H5C(NSiMe3)2]2Y(µµ-NH-C(CMe3)=C(H)Ph)}2 (3.11b). A solution of 2.7 (1.2 g,

1.5 mmol) in pentane (10 mL) was treated with N≡CCMe3 (0.14 mL, 1.5 mmol) at room temperature.

The yellow solution formed was allowed to stand overnight, after which it was concentrated to

approximately 5 mL and cooled to -80oC. The crude product was isolated and recrystallized from

pentane (5 mL). Crystallization at -80oC yielded 3.11b (0.9 g (1.1 mmol, 73 %) as a microcrystalline

material. 1H NMR (benzene-d6, δ): 7.80 (d, 2H, Ar, 3JH-H = 10 Hz), 7.25 (m, 6H, Ar), 7.09 (m, 7H,

Ar), 5.88 (s, 1H, NH), 5.25 (s, 1H, CH), 1.58 (s, 9H, C(CH3)3), 0.10 (s, 36H, C6H5C(NSi(CH3)3)2.13C NMR (benzene-d6, δ): 183.7 (s, C6H5C(NSiMe3)2), 170.1 (s, C(Me)=CH), 143.0 (s, Ar), 142.7(s,

Ar), 131.0 (dd, Ar, 1JC-H = 157 Hz, 2JC-H = 8 Hz), 128.0 (dd, Ar, 1JC-H = 161 Hz, 2JC-H = 6 Hz), 127.7

(dd, Ar, 1JC-H = 160 Hz, 2JC-H = 7 Hz), 126.5 (dt, Ar, 1JC-H = 157 Hz, 2JC-H = 7 Hz), 124.8 (d, Ar, 1JC-

H = 154 Hz), 122.1 (dt, Ar, 1JC-H = 156 Hz, 2JC-H = 7 Hz), 85.1 (d, CH, 1JC-H = 160 Hz), 37.8 (s,

C(CH3)3), 29.9 (q, C(CH3)3, 1JC-H = 125.5 Hz), 2.9 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 117 Hz).Anal.

Calcd. (found) for C76H124N10Si8Y2: C: 57.76 (58.10), H: 7.91 (8.04), Y: 11.25 (11.13).

NMR Tube Reaction of [C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) with N≡≡CMe. To an NMR tube

containing a solution of 2.7 (21 mg, 0.027 mmol) in benzene-d6 (0.5 mL), acetonitrile (3 µL, 0.05

mmol) was added at room temperature. The 1H NMR spectrum recorded after 10 minutes showed

that all 2.7 had reacted to give a mixture of compounds: 3.10 (10 %), {[C6H5C(NSiMe3)2]2Y(µ-

N=C(Me)CH2Ph)}2 (3.12a, 50 %) and {[C6H5C(NSiMe3)2]2Y(µ-NH-C(Me)=C(H)Ph)}2 (3.12b, 40 %)

along with resonances of uncoordinated THF. An 1H NMR spectrum recorded after two days at room

temperature showed no further changes. 1H NMR (benzene-d6) for 3.12a: 3.47 (s, 2H, CH2), 1.86 (s,

3H, CH3), 0.07 (s, 36H, C6H5C(NSi(CH3)3)2). 1H NMR (benzene-d6, δ) for 3.12b: 6.60 (s, 1H, NH),

4.78 (s, 1H, CH), 2.16 (s, 3H, CH3), 0.16 (s, 36H, C6H5C(NSi(CH3)3)2).

NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with N≡≡CMe. To an NMR tube

containing a benzene-d6 (0.5 mL) solution of 2.9 (20 mg, 0.032 mmol) N≡CMe was added (1.8 µL,

0.03 mmol). The 1H NMR spectrum monitored directly after the NMR tube was filled, showed

resonances characteristic for {[C6H5C(NSiMe3)2]2Y(µ-N=C(H)Me)}2 (3.13), δ: 9.3 (m, 1H,

Chapter 3.

74

N=C(H)Me), 7.28 (m, 4H, Ar), 7.08 (m, 6H, Ar), δ 2.40 (d, N=C(H)CH3, 3JH-H = 3.4 Hz), 0.11 (s, 36H,

C6H5C(NSi(CH3)3)2 and uncoordinated THF. Compound 3.13 is unstable and decomposes gradually

at room temperature to form a mixture of unidentified products.

NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with N≡≡CCMe3. To an NMR tube

containing a benzene-d6 (0.5 mL) solution of 2.9 (25 mg, 0.041 mmol), N≡CCMe3 was added (15 µL,

0.12 mmol). After 5 minutes at room temperature, the 1H NMR spectrum of the reaction mixture

showed characteristic resonances of uncoordinated THF and {[C6H5C(NSiMe3)2]2Y(µ-

N=C(H)CMe3)}2 (3.14). 1H NMR (benzene-d6, δ) for 3.14: 9.53 (m, 1H, N=C(H)Me, 7.31 (m, 4H, Ar),

7.07 (m, 6H, Ar), 1.25 (s, 9H, N=C(H)C(CH3)3), 0.19 (s, 36H, C6H5C(NSi(CH3)3)2. The product

(3.14) decomposes at room temperature to form a mixture of unidentified products.

NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with N≡≡CCMe3 and H2; Preparation

of [C6H5C(NSiMe3)2]2Y(N=C(CMe3)N(H)CH2CMe3) (3.15). N≡CCMe3 (16 µL, 0.14 mmol) was added

to a solution of 28 mg (0.045 mmol) of 2.9 in 0.5 mL of benzene-d6. The reaction mixture was

transferred into an NMR tube. At a vacuum line, the NMR tube was cooled to -198oC, degassed and

exposed to 1 atm. of H2. The progress of the reaction was followed by 1H NMR spectroscopy.

Initially, the same products (including 3.14) were formed as during the same reaction in the absence

of hydrogen. Gradually, the 1H NMR spectrum changed and after 1 day at room temperature, the

hydrogenated insertion product, [C6H5C(NSiMe3)2]2Y(N=C(CMe3)N(H)CH2CMe3) (3.15), had been

formed in 90 % yield. 1H NMR (benzene-d6, δ): 7.28 (m, 4H, Ar), 7.08 (m, 6H, Ar), 5.52, (s, 1H, NH),

3.40 (s, 2H, CH2), 1.19 (s, 9H, C(CH3)3), 1.17 (s, 9H, C(CH3)3), 0.12 (s, 36H, C6H5C(NSi(CH3)3)2.13C NMR (benzene-d6, δ): 186.6 (s, N(H)-C(CMe3)=NCH2CMe3), 183,5 (d, C6H5C(NSiMe3)2, 2JY-C =

3 Hz), 143.6 (s, Ar), 128.1 (dd, Ar, 1JC-H = 161 Hz, 2JC-H = 6 Hz), 127.8 (dd, Ar, 1JC-H = 161 Hz, 2JC-

H = 7 Hz), 126.4 (dt, Ar, 1JC-H = 157 Hz, 2JC-H = 7 Hz), 60.9 (d,CH2, 1JC-H = 133 Hz), 38.8 (s,

C(CH3)3), 32.9 (s, C(CH3)3), 29.2 (q, C(CH3)3, 1JC-H = 125 Hz), 28.6 (q, C(CH3)3, 1JC-H = 125 Hz),

2.7 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz).

NMR Tube Reaction of [C6H5C(NSiMe3)2]2YCH(SiMe3)2 (2.8) with Pyridine. Pyridine (6.6 µL,

0.08 mmol) was added to an NMR tube charged with 25 mg (0.032 mmol) of 2.8 in benzene-d6 (0.5

mL). The reaction was followed by 1H NMR. At room temperature, no reaction occurred. The NMR

tube was warmed to 50oC, upon which the solution changed from colorless to red. After one week at

50oC, the resonances of 2.8 had disappeared under formation of H2C(SiMe3)2 resonances and

signals in the vinyllic (3-6 ppm) and aromatic (7-9 ppm) regions.

NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with Pyridine. Pyridine (2.6 µL, 0.032

mmol) was added at room temperature to a solution of 2.9 (10 mg, 0.016 mmol) in benzene-d6 (0.5

mL). Upon addition, the solution changed from colorless to yellow. The reaction mixture was

transferred into an NMR tube and an 1H NMR spectrum was recorded. This showed that all 2.9 had

reacted under exclusive formation of the 1,2-insertion product [C6H5C(NSiMe3)2]2YNC5H6 (3.16). 1H

NMR (benzene-d6, δ): 6.54 (dd, 1H, H4 NC5H6, 3JH-H = 9.0 Hz, 3JH-H = 8.6 Hz), 5.44 (ddd, 1H, H5

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

75

NC5H6, 3JH-H = 5.6 Hz, 3JH-H = 6.0 Hz, 4JH-H = 2.4 Hz), 5.22 (dt, 1H, H3 NC5H6, 3JH-H = 9.4 Hz, 3JH-H

= 4.3 Hz), 4.52 (d, 2H, H2, H2’ NC5H6, 2JH-H = 4.3 Hz), 0.04, (s, 36H, C6H5C(NSi(CH3)3)2).

Preparation of [C6H5C(NSiMe3)2]2Y-NC5H5-2-CH2Ph. (3.17). Pyridine (0.15 mL, 1.8 mmol) was

added at room temperature to a solution of 2.7 (1.3 g, 1.7 mmol) in toluene (10 mL). The yellow

reaction mixture formed was stirred for 5 hours, after which the solvent was evaporated. The

remaining oil was stripped (3 x 5 mL) with pentane. Removal of all the volatiles in vacuo , resulted in

a sticky oil. The 1H NMR spectrum of the oil showed that 3.17 had been formed in 90 % yield.

Attempts to purify by sublimation or vacuum distillation failed and led to decomposition of the

product. 1H NMR (benzene-d6, δ): 7.02 (m, 1H, H6), 6.44 (dd, 1H, H4, 3JH-H = 9.0 Hz, 3JH-H = 8.6

Hz), 5.27 (dd, 1H, H5, 3JH-H = 6.0 Hz, 3JH-H = 4.7 Hz), 5.20 (dt, 1H, H3, 3JH-H = 6.0 Hz, 4JH-H = 2.6

Hz), 4.93 (dt, 1H, H2, 3JH-H = 6.0 Hz, 3JH-H = 5.1 Hz), 3.91 (d, 1H, C(H)H, 2JH-H = 11.5 Hz, 3JH-H =

11.0 Hz), 2.74 (dd, 1H C(H)H, 1JH-H = 12.0 Hz, 3JH-H = 8.1 Hz), 0.07 (s, 36H, C6H5C(NSi(CH3)3)2).13C NMR (benzene-d6, δ): 183.9 (s, C6H5C(NSiMe3)2), 143.1 (s, ipso-C ), 140.4 (d, Ar, 1JC-H = 161

Hz), 107.2 (d, CH NC5H4, 1JC-H = 160 Hz), 94.9 (dd, CH NC5H4, 1JC-H = 167 Hz, 3JC-H = 6 Hz), 55.4

(dt, C(H)CH2Ph, 1JC-H = 34.7 Hz, 3JC-H = 6 Hz), 41.6 d, CH2, 1JC-H = 29 Hz), 2.5 (q,

C6H5C(NS(CH3)3)2, 1JC-H = 119 Hz).

Preparation of [C6H5C(NSiMe3)2]2Y(ηη2-(C,N)-CH2-2-NC5H4) (3.18). To a solution of 2.7 (1.4 g,

1.8 mmol) in toluene (20 mL), α-picoline (180 µL, 1.8 mmol) was added at room temperature. After 4

hours, the volatiles were removed in vacuo and the remaining orange oil was stripped with pentane

(3 x 5 mL) but the product did not solidify. An 1H NMR spectrum of the isolated crude product (1.2 g,

1.5 mmol, 84%) showed some traces of 2.7 and α-picoline as the only contaminations. Since

crystallization, sublimation and vacuum distillation failed, the product could not be isolated

analytically pure. 1H (benzene-d6, δ): 7.92 (d, 1H, Ar, 3JH-H = 5.6 Hz), 7.30 (m, 4H, Ar), 7.02 (m, 6H,

Ar), 6.87 (ddd, 1H, H4 NC5H4, 3JH-H = 8.6 Hz, 3JH-H = 7.1 Hz, 4JH-H = 1.2 Hz), 6.68 (d, 1H, H3

NC5H4, 3JH-H = 8.3 Hz), 6.07 (dd, 1H, H5 NC5H4, 3JH-H = 6.8 Hz, 3JH-H = 5.6 Hz), 2.71 (d, 2H, YCH2,2JY-H = 0.9 Hz), 0.05 (s, 36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 183.8 (s,

C6H5C(NSiMe3)2), 166.3 (s, NC5H4), 146.3 (d, NC5H4, 1JC-H = 172 Hz), 135.6 (dd, NC5H4, 1JC-H =

158 Hz, 3JC-H = 7 Hz), 127.7 (d, Ar, 1JC-H = 161 Hz), 126.3 (d, Ar, 1JC-H = 157 Hz), 120.4 (d, Ar, 1JC-

H = 163 Hz), 106.9 (d, Ar, 1JC-H = 163 Hz), 52.4 (dt, YCH2, 1JC-H = 143 Hz, 1JY-H = 6 Hz), 2.6 (q,

C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz).

References and Notes.

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Chapter 3.

76

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Bis(Benzamidinato) Yttrium Compounds: Reactivity.

77

9 Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.; Alvarez, D. Jr.;

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11 The polyethylene was characterized by IR.

12 Mw = 239900, Mn = 46300, Mw/Mn = 5.2. Note that the broad Mw/Mn value cannot be explained

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Termination by β-H elimination will exacerbate the Mw/Mn broadening. Furthermore, for

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13 (a) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471-6473. (b) Eshuis, J. J. W.

Ph. D. Thesis , University of Groningen, 1991.

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18 Horton, A. D. J. Chem. Soc., Chem. Commun. 1992, 185-187.

19 Duchateau, R.; Meetsma, A.; Teuben, J. H. manuscript in preparation.

20 (a) Evans, W. J.; Drummond, D. K.; Hanusa, T. P.; Olofson, J. M. J. Organomet. Chem.

1989, 376, 311-320. (b) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993, 12,

2618-2633.

21 For example see: (a) Evans, W. J.; Wayda, A. L. J. Organomet. Chem. 1980, 202, C6-C8.

(b) Evans, W. J.; Engerer, W. J.; Coleson, K. M. J. Am. Chem. Soc. 1981, 103, 6672-6677.

(c) Atwood, J. L.; Hunter, W. E.; Wayda, A. L.; Evans, W. J. Inorg. Chem. 1981, 20, 4115-

4119. (d) Boncella, J. M.; Tilley, T. D.; Andersen, R. A. J. Chem. Soc., Chem. Commun.

1984, 710-712. (e) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. Organometallics

1983, 2, 709-714. (f) Shen, Q.; Zheng, D.; Lin, L.; Lin, Y. J. Organomet. Chem. 1990, 391,

307-312.

22 HC≡CH: ν(C≡C) (Raman) = 1947 cm-1; HC≡CMe: ν(C≡C) = 2140 cm-1; HC≡CPh ν(C≡C) = 2110 cm-

1; HC≡CCMe3: ν(C≡C) = 2110 cm-1; HC≡CSiMe3: ν(C≡C): 2037 cm-1.

23 Crystal data for (C28H47N4Si4Y)2: Space group: monoclinic, C2/c with a = 27.088(2) Å, b =

27.102(2) Å, c = 20.964(1) Å, β = 113.40(1)o, V = 14125(2) Å3, dcalc = 1.206 g.cm-3, and Z = 8.

Data were collected on an Enraf-Nonius CAD-4F diffractometer at 130 K with Mo K-α (λ =

0.71073 Å). The structure was solved by Patterson methods and refined to R(F) = 0.044,

Rw(F) = 0.036 for 7813 unique reflections with I ≥ 2.5 σ(I) and 1035 parameters.

24 Erker, G.; Frömberg, W.; Benn, R.; Mynott, R.; Angermund, K.; Krüger, C. Organometallics

1989, 8, 911-920.

Chapter 3.

78

25 Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L. Hunter, W. E. J. Chem.

Soc., Dalton Trans. 1979, 54-61.

26 (a) Morrison, T. T.; Boyd, R. N. In Organic Chemistry ; Allyn and Bacon: Boston, 1980, p 250.

(b) Tsutsui, M.; Ely, N.; Gebala, A. Inorg. Chem. 1975, 14, 78-81.

27 (a) C≡C distances for free acetylenes range from 1.19-1.21 Å: Dale, J. In Chemistry of

Acetylenes , Viehe, H. G., Ed., Marcel Dekker: New York, 1969, pp 3-96. (b) Weiss, E.; Plaso,

H. Chem. Ber. 1968, 101, 2947-2955. (c) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.;

Ziller, J. W.; Alvarez, D. Jr. Organometallics 1990, 9, 2124-2130.

28 (a) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1990, 9, 2628-2631. (b) Heeres,

H. J.; Nijhof, J.; Teuben, J. H. Organometallics 1993, 12, 2609-2617. (c) Forsyth, C. M.;

Nolan, S. P.; Stern, C. L.; Marks, T. J., Reingold, A. L.Organometallics 1993, 12, 3618-3623.

29 (a) St. Clair, M.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1991, 10, 525-527. (b)

Evans, W. J.; Rabe, G. W.; Ziller, J. W. J. Organomet. Chem. 1994, 483, 21-25.

30 Heeres, H. J. Ph. D. Thesis , University of Groningen, 1990.

31 Stockis and Hoffman have performed calculations on the polarization of the π* orbitals of

HC≡CMe and HC≡CSiMe3. The acetylene substituent appears to have a pronounced influence

on the electron density of the alkyne sp-carbon atoms and distinct differences in polarization

were found for both alkynes. These electronic effects are believed to be responsible for the

difference in regioselectivity of the catalytic dimerization of 1-alkynes. Stockis, A.; Hoffman,

R. J. Am. Chem. Soc. 1980, 102, 2952-2962.

32 Cp2YCH(SiMe3)2, prepared from {Cp2YCl}n and LiCH(SiMe3)2 in ether, was treated with 10 fold

excess of HC≡CCMe3. Heating to 80oC for two weeks showed the formation of {Cp2Y(µ-

C≡CCMe3)}2 (ref: 1b) as the only product, which does not show any reactivity with excess of

HC≡CCMe3. Duchateau, R.; Teuben, J. H. unpublished results.

33 The steric bulk of the bis(benzamidinato) ligand system is slightly smaller than of the

bis(pentamethylcyclopentadienyl) ligand set but significantly larger than of the

bis(cyclopentadienyl) ligand environment. The yttrium center in bis(benzamidinato) complexes

was found to be considerable more electrophilic than in the corresponding yttrocene

derivatives. For details see Chapter 7.

34 (a) Bercaw, J. E.; Davies, D.; Wolczanski, P. T. Organometallics 1986, 5, 443-450. (b)

Patten, T. E.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 5065-5066.

35 (a) Binev, I. G.; Todorova-Momcheva, R. I.; Juchnovski, I. N. Doklady Bolgarskoj Academii

Nauk. 1976, 29, 1301-1304. (b) Juchnovski, I. N.; Binev, I. G. J. Organomet. Chem. 1975,

99, 1-10.

36 Krüger, C. J. Organomet. Chem. 1967, 9, 125-134.

37 (a) Guram, A. S.; Jordan, R. F.; Taylor, D. F. J. Am. Chem. Soc. 1991, 113, 1833-1835. (b)

Borkowsky, S. L.; Baenziger, N. C.; Jordan, R. F. Organometallics 1993, 12, 486-495. (c)

Alelyunas, Y. W.; Guo, Z.; LaPointe, R. E.; Jordan, R. F. Organometallics 1993, 12, 544-553.

(d) Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics 1990, 9, 1546-1557.

38 Hirabayashi, T.; Itoh, K.; Sakai, S.; Ishii, Y. J. Organomet. Chem. 1970, 21, 273-280.

Bis(Benzamidinato) Yttrium Compounds: Reactivity.

79

39 3.15 is assumed to be similar to the scandium analogue. However, on the bases of NMR data

only, no distinction can be made between the following two possible structures:

Y

HR

H

H

C

RC

N

N

Y

H

R

H

C

RC

N

N

H

40 (a) Deelman, B.-J.; Stevels, W. M.; Lakin, M. T.; Spek, A. L.; Teuben, J. H. Organometallics

1994, 13, 3881-3891. (b) Deelman, B.-J. Ph. D. Thesis , University of Groningen, 1994.

41 (a) March, J. Advanced Organic Chemistry, Reactions, Mechansims and Structure , 3rd

ed.; John Wiley and Sons: New York, 1985. (b) Joule, J. A.; Smith, G. F. Heterocyclic

Chemistry ; Van Nostrand Reinhold Company: London, 1972.

42 Colgan, D.; Papasergio, R. I.; Raston, C. L.; White, A. H. J. Chem. Soc., Chem. Commun.

1984, 1708-1710.